The invention relates generally to techniques for processing materials in supercritical fluids. Specifically, embodiments of the invention include techniques for controlling parameters associated with a material processing capsule disposed within a high-pressure apparatus/enclosure.
Supercritical fluids may be used to process a wide variety of materials. A supercritical fluid is defined as a substance beyond its critical point, i.e., critical temperature and critical pressure. A critical point represents the highest temperature and pressure at which the substance can exist as a vapor and liquid in equilibrium. In certain supercritical fluid applications, the materials being processed are placed inside a pressure vessel or other high pressure apparatus. In some cases it is desirable to first place the materials inside a container, liner, or capsule, which in turn is placed inside the high pressure apparatus. In operation, the high pressure apparatus provides structural support for the high pressures applied to the container or capsule holding the materials. The container, liner, or capsule provides a closed/sealed environment that is chemically inert and impermeable to solvents and gases that may be generated by the process. In some applications, such as crystal growth, the pressure vessel or capsule also includes a baffle plate that separates the interior into different chambers, e.g., a top half and a bottom half. The baffle plate typically has a plurality of random or uniformly spaced holes to enable fluid flow and heat and mass transfer between these different chambers, which hold the different materials being processed along with a supercritical fluid. For example, in typical crystal growth applications, one half of the capsule contains seed crystals and the other half contains nutrient material. In addition to the materials being processed, the capsule contains a solid or liquid that forms the supercritical fluid at elevated temperatures and pressures and, typically, also a mineralizer to increase the solubility of the materials being processed in the supercritical fluid. In other applications, for example, synthesis of zeolites or of nano-particles or processing of ceramics, no baffle plate may be used for operation. In operation, the capsule is heated and pressurized toward or beyond the critical point, thereby causing the solid and/or liquid to transform into the supercritical fluid.
The processing limitations for conventional steel hot-wall pressure vessels (e.g., autoclaves) are typically limited to a maximum temperature of about 400 Degrees Celsius and a maximum pressure of 0.2 GigaPascals (GPa). Fabrication of pressure vessels from nickel-based superalloys allows for operation at a maximum temperature of about 550 degrees Celsius and a maximum pressure of about 0.5 GPa. Therefore, these hot-wall pressure vessels are inadequate for some processes, such as the growth of gallium nitride crystals in supercritical ammonia, which require pressures and temperatures that extend significantly above this range in order to achieve growth rates above about 2-3 microns per hour.
In addition, existing cold-wall pressure vessels, e.g., hot isostatic presses (HIPs), may not adequately account for pressure differences between the interior of the capsule and the high pressure apparatus surrounding the exterior of the capsule. For example, during the growth of crystals, the capsule inside the high pressure apparatus tends to deform due to the difference in pressure between an inner surface and an outer surface of the capsule. Moreover, existing baffle plates within the capsule or pressure vessel have random holes, which do not adequately distribute the heat and substances circulating within the capsule.
Accordingly, a technique is needed for processing materials with supercritical fluids at relatively higher temperatures and pressures than existing systems. A need also exists for monitoring and controlling the environment inside the capsule to facilitate a uniform growth of crystals. A further need exists for monitoring and controlling the differential pressures on the walls of the capsule to reduce deformations of the capsule. Also, an improved baffle plate is needed to provide a desired heat distribution and flow profile within the capsule to, thereby, provide a uniform growth of crystals.